I have a 23 model f350 lariat ultimate and the reverse lights have stopped working. Replaced one blown fuse under the hood which was #21 for trailer reverse lights. Checked every single fuse in both junction boxes can't seem to find another blown fuse. Does anyone know if I may be missing something

 

If your vehicle is a std shift it should have a reverse light sw on the trans that usually screws into a hole in the casting.

An automatic usually uses the switch on the driver's side or top of the trans.
A simple thing like a test light helps checking things like this a lot easier.